Essential to industrial processing of concentrated electrolytes that exist as nuclear waste (Hanford site, WA) or Bayer liquers alumina (Bayer process) is understanding the chemical transformations of Al(III). Herein, Al(OH)4- is the predominant aqueous aluminum species under basic conditions, with Raman spectroscopy also supporting the presence of the dimeric aluminate species Al2O(OH)62-. No indisputable evidence exists for other dimeric or higher-order oligomeric species in these conditions, which hinders the understanding of the early stages of nucleation and crystallization of aluminum hydroxides and oxyhydroxides (e.g., gibbsite, boehmite). In this work, we explored the free energy profile of the dimerization by means of umbrella sampling density functional tight-binding using the monomer-monomer distance as a biasing collective variable. At infinite dilution, Al2(OH)82- is formed and aluminate-sodium contact ion pairing interactions have no effect on the energetics of the dimerization itself. However, even at moderate NaOH concentrations the Al2O(OH)62-dimer is systematically formed rather than Al2(OH)82-, consistently with the decreasing water activity. For a given reaction pathway, the NaOH concentration influences the reaction and activation energies, due to the varying charge transfer and polarization contributions.

Exploring the role of electrolyte concentration in aluminate dimerization

2019 Fall National Meeting

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